What is annealing? Why is it necessary?

One of the most distinctive physical properties of glass is how it reacts to temperature variation. When it heats, it expands; and as it cools, it contracts. These processes create stresses within the glass, especially between the interior and the surface where the widest temperature variations can be found.

To relieve stresses, which can lead to breakage at room temperature, it is necessary to cool glass in a controlled manner through a predetermined temperature gradient. This allows the surface and interior to cool uniformly. This controlled process for cooling glass to relieve interior stresses is called "annealing."